The present disclosure has as its object a brake control unit including a primary piston defining a primary chamber connected to a primary circuit and a secondary piston defining a secondary chamber connected to a secondary circuit, the control unit receiving the braking demand signal supplied via the push-rod actuated by the driver, said demand signal being detected by the displacement sensor for the movement of the push-rod in order to control the primary piston, which itself pushes against the secondary piston, thereby generating a pressure inside the primary chamber which then exerts a hydraulic thrust on the rear cross section of the secondary piston.
Generally, the disclosure has as its object a brake control unit which corresponds substantially to a tandem master cylinder and is intended, for example, for hybrid vehicles. The primary chamber is connected to the hydraulic circuit for the brakes of the rear axle, which is not a driving axle. The secondary chamber is connected in a decoupled manner to the braking circuit of the front axle. The front axle is a driving axle. It is equipped with electric motors ensuring regenerative braking (dynamic braking) in combination with mechanical braking via brakes that are controlled by a braking circuit. However, the braking circuit for the front axle is not connected directly to the secondary pressure chamber which acts indirectly. Braking of the front axle is assured, initially, principally by the dynamic braking, the electric motors functioning as a generator and from a certain speed down to zero speed, the mechanical braking being combined with the dynamic braking before replacing it completely when the speed falls below 10 km/h, for example, and continuing down to 0 km/h.
It should be noted that the pressure generated inside the secondary chamber of a tandem master cylinder results from the thrust generated by the primary pressure acting on the hydraulic cross section of the rear part of the secondary piston, said thrust going on to generate a pressure inside the secondary chamber according to the hydraulic cross section of the front part of the secondary piston. The hydraulic cross sections of the front and rear parts of the secondary piston usually being of the same value, the primary and secondary pressures of a traditional master cylinder are substantially the same.
Generally, certain recent braking systems require a master cylinder having a bore of small dimensions for multiple reasons, such as the characteristics in degraded mode or the feel through the pedal. However, producing bores of reduced cross sections in master cylinders and grooves for cups of small dimensions is a difficult and costly machining operation. Furthermore, cups (or lip seals) of small dimensions of the kind that would be necessary do not exist in the standard manufacturing ranges. These special seals would be difficult to develop, to manufacture and to install in the throat surrounding the bore, by comparison with the installation of lip seals with a large diameter.
In addition, machining throats in bores with a small diameter is a difficult and costly operation.
FIG. 1 is a simplified diagram of a tandem master cylinder, of a known kind, for the purpose of describing the definition of the characterizing feature connecting the inlet force FIN, applied by the push-rod to the primary piston P1 and the pressure Ps inside the secondary chamber CH2.
The tandem master cylinder has a primary piston P1, of which the front cross section S1 defines the primary chamber CH1 which is further defined by the rear cross section S2 of the secondary piston P2.
The front cross section S2b of the secondary piston P2 defines the secondary chamber CH2.
Under the effect of the force FIN, a primary pressure Pp is present inside the primary chamber CH1, which pressure Pp, when applied to the secondary rear cross section S2 of the secondary piston P2, generates a force Fs displacing the secondary piston, of which the front cross section S2b generates the secondary pressure Ps inside the secondary chamber CH2.
By definition, the relationship between the inlet force FIN and the secondary pressure PS is as follows:
  Ps  =            F      IN        k  being an equation in which the coefficient k has the dimension of a surface; this coefficient is referred to by convention as the “equivalent cross section Se”, such that:
                    Ps        =                              F            IN                    Se                                    (        1        )            
However, in the master cylinder defined above, the primary pressure Pp inside the primary chamber CH1 is given by the following equation:
                              P          p                =                              F                          I              ⁢                                                          ⁢              N                                            S            ⁢                                                  ⁢            1                                              (        2        )            
This primary pressure gives rise to the force Fs exerted on the secondary piston P2 according to the equation:
                                          F            ⁢                                                  ⁢            s                    -                                    P              p                        ·                          S              2                                      =                                            F                              I                ⁢                                                                  ⁢                N                                                    S              1                                ·                      S            2                                              (        3        )            
The force Fs generates the pressure Ps inside the secondary chamber CH2:
                              P          ⁢                                          ⁢          s                =                                            F              ⁢                                                          ⁢              s                                      S                              2                ⁢                                                                  ⁢                b                                              =                                    F              IN                        ·                                          S                2                                                              S                  1                                ·                                  S                                      2                    ⁢                                                                                  ⁢                    b                                                                                                          (        4        )            
The equivalent cross section SE may accordingly be described as follows:
      P    ⁢                  ⁢    s    =                                          F                          I              ⁢                                                          ⁢              N                                            S            ⁢                                                  ⁢            e                          ⁢                  (          1          )                    →              S        ⁢                                  ⁢        e              =                            F                      I            ⁢                                                  ⁢            N                                    P          ⁢                                          ⁢          s                    =                                    S            1                    ·                      S                          2              ⁢                                                          ⁢              b                                                S          2                    
The following equation is thus obtained:
                              S          ⁢                                          ⁢          e                =                                            S              1                                      S              2                                ·                      S                          2              ⁢                                                          ⁢              b                                                          (        5        )            
The presentation made above with the cross sections of the pistons, according to normal practice in the field of master cylinders, gives the diameter of the pistons or the seals (cups) according to the following traditional formula connecting the surface of a circular disc to its diameter D:
                    S        =                              π            ×                                          D                2                                            2                2                                              =                                    π              4                        ×                          D              2                                                          (        6        )            or also:
                    D        =                  2          ⁢                                    S              π                                                          (                  6          ⁢          bis                )            